Air compressor systems are used on railway locomotives to develop compressed air for operating various elements of a locomotive and in particular for supplying compressed air for operating air braking equipment. The typical system includes a two-stage compressor with an intercooler between the stages, an aftercooler connected to receive compressed air from a high pressure stage of the compressor, a shrouded fan to force ambient cooling air over the intercooler and aftercooler, and an air reservoir connected for receiving the cooled compressed air from the aftercooler. The aftercooler is required to lower the temperature of the compressed air since the elevated temperature caused by compression can reach levels that may cause damage to the braking equipment or other equipment to which the air is being supplied. In addition, the higher temperature compressed air entrains more moisture which precipitates out as condensation as the air is cooled and needs to be removed from the air in order to protect the air equipment from moisture damage. The aftercooler condenses the moisture in the air forming condensation which is then blown through the passages of the aftercooler by flow of the compressed air and deposited in an air storage reservoir connected to the outlet of the aftercooler. The air storage reservoir generally includes a manual and an automatic drain cock through which the accumulated condensation can be expelled.
When the external or ambient air temperature falls below the freezing point of water, the condensate may freeze in the passages of the aftercooler before it can be swept into the reservoir. Such freezing generally occurs if the ambient air temperature falls to about -10.degree. F. in some locomotive applications but may occur at any temperature below the freezing point of water depending on the aftercooler location and efficiency. When this occurs, at least some of the aftercooler passages may become blocked by ice and inhibit the flow of air through the aftercooler and to the air storage reservoir. In such event, there is a risk that the air pressure at the air reservoir may fall to less than that necessary to operate the air brake equipment and force the locomotive to be removed from service. Further, the air supply system includes a pressure relief valve between the aftercooler and air compressor which can be tripped by excess air pressure caused by the reduced air flow through the aftercooler, increasing the risk that the compressor will be unable to supply sufficient air to maintain an operative air brake system. Accordingly, it would be desirable to provide a method and apparatus which overcomes the likelihood of air pressure loss caused by blockage of the aftercooler.